MEV-Boost

MEV-Boost was released by Flashbots around the Merge in 2022 as an out-of-protocol version of proposer-builder separation. A validator runs it next to its consensus client and connects to one or more relays. When the validator is due to propose, the relays forward the highest bids from builders; the validator signs a blinded block header, and only then does the relay reveal the full block for publication.

Adoption grew quickly because it raises staking rewards: a validator using MEV-Boost earns the builder's payment on top of normal rewards, without running MEV strategies itself. A large majority of Ethereum blocks pass through it, and staking services compete partly on which relays they connect to and how they share MEV income with users.

Relays are trusted middlemen. A faulty or malicious relay could withhold a block after the validator signs, costing the proposer its slot, and relays can filter transactions. Relay operators are few, have no protocol-level incentives, and some apply sanctions filtering. Enshrined proposer-builder separation is meant to make that trust unnecessary.